1. Field of the Invention
This invention relates to a method of manufacturing a variable-capacitance diode device which can be most effectively used as a tuning element, and more particularly it pertains to such a method in which the profile of impurity concentration of the diode device can easily be controlled depth-wise of the surface of the device.
2. Description of the Prior Art
Variable-capacitance diode devices have recently found extensive use as tuning elements for electronic tuning circuit. There have conventionally been demands for variable-capacitance diode devices with a wide voltage range. With a variable-capacitance diode device, as the potential difference across the junction thereof increases with a reverse bias voltage (referred to as applied voltage hereinafter), the width of depletion layer tends to expand with increasing impurity concentration in the semiconductor layer. To achieve a variable-capacitance diode device with a wide voltage range, it has been the usual practice that the impurity concentration in that region of the semiconductor layer where the depletion layer tends to expand, is controlled so as to be proximate to the Gaussian distribution, i.e., so as to decrease smoothly so that the expansion of the depletion layer increases gradually with the applied voltage. Recently, however, there have been demands for a variable-capacitance diode device which is designed such that a wide range of variation in the depletion layer width occurs with respect to the narrow range of variation in the applied voltage and sufficient tuning capacitance is available.
A conventional method of making a variable-capacitance diode device will now be described with reference to FIG. 1 wherein the abscissa represents the depth X.sub.i from the surface of the semiconductor substrate of the diode device, and the ordinate indicates the impurity concentration C on semi-logarithmic scale.
In FIG. 1, the dotted curve (10) represents the profile of impurity concentration of an N.sup.+ conductivity type semiconductor layer formed by means of an ion-implantation process. Formed in the N.sup.+ conductivity type semiconductor layer is a P.sup.+ conductivity type semiconductor layer also by means of an ion-implantation process, the profile of impurity concentration thereof being indicated by the solid curve (11). In this way, a PN junction J is defined between the N.sup.+ and P.sup.+ type semiconductor layers, and the variable-capacitance diode device is made. The profile of impurity concentration in the N.sup.+ conductivity type semiconductor layer as indicated by the curve (12) decreases smoothly except for the PN junction J, and it is usual that the concentration curve (12) represents a concentration profile in the form of Gaussian distribution which is proximate to the curve (10). In FIG. 1, the region indicated at (13) corresponds to an epitaxial layer, and the region shown at (14) corresponds to substrate.
However, it will be seen that the profile of impurity concentration in the N.sup.+ conductivity type semiconductor layer, except in the proximity to the PN junction J, is such that the following relationship holds: EQU A.sub.i &gt;A.sub.i+1
on the assumption that the impurity concentration at the highest point of the curve is A1, and those at sequential points are A.sub.2, A.sub.3, . . . , A.sub.i, A.sub.i+1, . . . , respectively. The profile of impurity concentration in the region A.sub.1, A.sub.2, A.sub.3 appears approximate to the Gaussian distribution, and tends to swell out. Such swelling will be described with reference to FIG. 2 which illustrates the relationship between the applied voltage and the capacitance on semi-logarithmic scale.
As shown at (I) in FIG. 2, the relationship between the applied voltage and the capacitance varies along a curve resembling inverted S-shape corresponding to the profile of impurity concentration, instead of changing linearly from the maximum impurity concentration C.sub.max to the minimum impurity concentration C.sub.min. Thus, when the voltage range over which the variable-capacitance diode device is usable as tuning element is wide, critical problems are relatively less likely to arise. In contrast thereto, when it is desired that a variable-capacitance diode device having a similar construction to that of the prior art be operated with an applied voltage as low as 1 or 2 V, i.e., when it is attempted to make use of tuning capacitance occurring over a narrow voltage range as shown at (II) in FIG. 2, there occurs such a disadvantage that sufficient capacitance variation is not available, and hence it is necessary to improve the characteristics of the variable-capacitance diode device.